There are extensive deposits of viscous hydrocarbons throughout the globe, including large deposits in the Alberta tar sands and in Venezuela, which are not recoverable with traditional oil well drill and pump production technologies. The problem with producing hydrocarbons from such deposits is that the hydrocarbons are too viscous to flow at commercially viable rates at typical reservoir temperatures and pressures. In some cases, these deposits are mined using open-pit mining techniques to extract the hydrocarbon-bearing material for later processing to extract the hydrocarbons. However, many deposits cannot be mined in this way and other methods are needed.
An alternative to open-pit mining is to heat the heavy oil to reduce its viscosity until it is pumpable. A variety of thermal techniques are used to heat the reservoir fluids and rock to produce the heated, mobilized hydrocarbons from wells. One such technique for utilizing a single well for injecting heated fluids and producing hydrocarbons is described in U.S. Pat. No. 4,116,275, which also describes some of the problems associated with the production of mobilized viscous hydrocarbons from horizontal wells.
Another thermal method of recovering viscous hydrocarbons is known as steam-assisted gravity drainage (SAGD) and is currently the only commercial process that allows for the extraction of bitumen at depths too deep to be strip-mined. Various embodiments of the SAGD process are described in CA1304287 and corresponding U.S. Pat. No. 4,344,485.
In SAGD, a vertical well is drilled and connected to at least two horizontal wells that are parallel and placed some distance apart, one above the other, and near the bottom of a payzone. Steam is pumped through the upper, horizontal injection well into a viscous hydrocarbon reservoir to heat or otherwise reduce the viscosity of the heavy oil, which can then drain to the lower well for collection.
The SAGD process is believed to work as follows. The injected steam creates a “steam chamber” in the reservoir around and above the horizontal injection well. As the steam chamber expands from the injection well, viscous hydrocarbons in the reservoir are heated and mobilized, especially at the margins of the steam chamber where the steam condenses and heats a layer of viscous hydrocarbons by thermal conduction. The heated, mobilized hydrocarbons (and steam condensate) drain under the effects of gravity towards the bottom of the steam chamber, where the production well is located. The mobilized hydrocarbons are thus collected and produced from the production well.
In order to initiate a SAGD production, thermal or fluid communication must be established between an injection and a production SAGD well pair. Initially, the steam injected into the injection well of the SAGD well pair will not have any effect on the production well until at least some thermal communication is established because the hydrocarbon deposits are so viscous and have little mobility. Accordingly, a start-up phase is required for the SAGD operation. Typically, the start-up phase takes about three months before thermal communication is established between the SAGD well pair, depending on the formation lithology and the actual inter-well spacing.
The traditional approach to starting-up the SAGD process is to simultaneously operate the injection and production wells independently of one another to circulate steam. The injection and production wells are each completed with a screened (porous) casing (or liner) and an internal tubing string extending to the end of the liner, forming an annulus between the tubing string and casing. High pressure steam is simultaneously injected through the tubing string of both the injection and production wells. Fluid is simultaneously produced from each of the injection and production wells through the annulus between the tubing string and the casing. In effect, heated fluid is independently circulated in each of the injection and production wells during the start-up phase, heating the hydrocarbon formation around each well by thermal conduction.
Independent circulation of the wells is continued until efficient communication between the wells is established. In this way, an increase in the fluid transmissibility through the inter-well span between the injection and production wells is established by conductive heating. This pre-heating start up stage typically takes about three to four months. Once sufficient thermal communication is established between the injection wells, the upper, injection well is dedicated to steam injection and the lower, production well is dedicated to fluid production.
What is needed in the art are methods to improve the efficiency and cost effectiveness of the above start up process for various gravity drainage techniques.